They then hid in the toilets at Old Trafford rather than exiting with the rest of the tour group, and were only spotted the morning after.

United say there was no risk to safety, with all those who go on stadium tours searched beforehand with metal detectors, as well as their belongings, and counted in and out by tour guides.

United have launched an internal investigation with alarm bells triggered that two members of the public could remain undetected inside Old Trafford only hours before a 75,000 sell-out Premier League game.

The police were called but no arrests were made.

A United spokesman said: "We realise people will go to any lengths to watch a game at Old Trafford but these people went too far. The fact that they and their belongings had been searched before entry means we are confident there was no risk to safety."

It is the second breach of security the Red Devils have suffered this year after the final league game of last season was postponed after a bomb – later found to be a dummy device left behind after a training exercise – was discovered in a toilet.

The Daily Mail were told: “You honestly could not make it up. The two have gone on the tour on the Friday and for some reason, they must not have counted the numbers in and out and they have stayed there overnight without anyone seeing them.

“After the Bournemouth fiasco they retrained all the staff and beefed up security and searches but then this happens.”